1. Field of the Invention
This invention relates to an improvement in an electrophotography using a magnetic brush developing method for performing reversal development with a one-component magnetic toner.
2. Description of the Prior Art
It is well known that a photographic positive of an image to be reproduced is obtained by means of magnetic brush development with a one-component toner (positive development). Specifically, first, an electrophotosensitive material is electrostatically charged; the so electrostatically charged sheet is exposed to an image to be reproduced, thereby forming a corresponding electrostatic latent image, and finally, toner particles are applied according to the magnetic brush developing method to the part of the photosensitive sheet on which electric charges still remain (i.e. unilluminated part of the photosensitive sheet). This kind of positive development is disclosed, for instance, in Japanese Patent Publication No. 56(1981)-2705.
Also, it is well known that a photographic negative is obtained according to the magnetic brush developing method using a one-component toner (reversal development). After forming an electrostatic latent image on an electrophotosensitive material in the same way as mentioned above toner particles are applied to the illuminated part or electric charge-free part of the material according to the magnetic brush developing method. While developing almost the same bias voltage as is applied to the photosensitive sheet is applied to a development sleeve of a developing device. This reversal development is disclosed in Japanese Patent Publication No. 56(1981)-2705 and Japanese Unexamined Patent Publication No. 52(1977)-146243, 53(1978)-112740, 53(1978)-115299 and 54(1979)-98248.
The experiments which the present inventors made, revealed that when a photographic reversal image was obtained by developing an electrophotosensitive material composed, for instance, of a dispersion of pulverized zinc oxide in an appropriate binder, a relatively extensive fog appeared in the reversal photographic image, and it had poor contrast, compared with a photographic positive image.
Japanese Unexamined Patent Publication No. 55(1980)-134864 discloses an improved magnetic brush developing method according to which a reversal photographic image having reduced fog and good contrast can be obtained. According to the disclosed method an electrophotosensitive material is electrostatically charged to a relatively low potential compared with the potential which would appear on the electrophotosensitive sheet when electrostatically charged to its full capacity (hereinafter referred to as "saturation-charging potential"). For instance, the material is electrostatically charged to a potential as low as 60 percent of the saturation-charging potential. When developing, a potential of the same polarity and same value as (or somewhat larger value than) the potential appearing on the so charged material, is applied to a one-component toner powder.
This method proposed and for which patent has been applied for assures production of reversal photographic images of the same quality as positive photographic images obtained by magnetic brush development using a one-component magnetic toner. The method, however, has some defects as follows; it requires a "scorotron", or a charged ion-generating apparatus having a main power supply for corona wires and a sub-power supply for grid electrodes, which is designed for controlling the amount of electric charges emitted from the corona wires. Thus, the apparatus which is used in carrying out the proposed method is very expensive and large in size.
In operation it is necessary to electrostatically charge electrophotosensitive material to a predetermined potential or higher. Disadvantageously, a "scorotron" requires an extended length of time for electrostatic charging, compared with a "corotron", and therefore the use of "scorotron" is a detriment to the speed-up of development. In an attempt to expedite development a combination of a "scorotron" with a "corotron" is proposed in Japanese Unexamined Patent Publication No. 55(1980)-144260. With recourse to the proposed combination, however, the time involved for development cannot be shortened enough. Another disadvantage is that the proposed apparatus inevitably becomes very complicated and larger in size compared with the use of a "corotron" above.